The present invention relates to internal shoe drum brakes with a slack adjuster.
The invention is particularly concerned with internal shoe drum brakes of the class having a pair of expansible brake shoes, shoe return spring means for retracting the shoes, power actuated service brake means for expanding the shoes, manually actuated auxiliary brake means for expanding the shoes, a strut of adjustable length which transmits manually applied loads between the shoes, the service brake means being operable to relieve the strut of load, the strut comprising a pair of relatively rotatable members screw threaded together, and a pivoted adjusting lever for effecting relative rotation of the strut members to lengthen the strut upon brake application by the auxiliary brake means.
In brakes of this class, it is known to arrange for the adjusting lever to be driven through the intermediary of a torsion spring.
There are two procedures for adjusting shoe clearances, one being that the auxiliary brake means is actuated to apply brake without any relief of load on the strut by operation of the service brake means, the other being that after the load on the strut is relieved upon brake application by the service brake means the auxiliary brake means is actuated to apply brake.
With the former procedure, it is easy to adjust shoe clearances because such adjustment is effected by the auxiliary brake means only. Therefore, this procedure lends itself to adjustment of relatively large shoe clearances. However, a large manual force is required upon brake application by the auxiliary brake means to overcome a rolling resistance opposing relative rotation of one of the strut members, in the form of the ratchet wheel, to the other and a load on the strut by shoe return spring means.
With the latter procedure, it is not easy to adjust relatively large shoe clearances because the service brake means must be operated before shoe clearance adjustment is effected by the auxiliary brake means. However, this prodecure lends itself to adjustment of relatively small shoe clearances upon automatic wear compensation because with a small force the auxiliary brake means can be actuated.